Lubricating oil for a rolling bearing in a high-speed rotating equipment

ABSTRACT

To provide a lubricating oil which is used for a high-speed rotating equipment such as a high-speed dental air turbine hand piece and is excellent in biosafety (reduced harm to human body), environmental conservation (safety to the environment), heat resistance (autoclaving resistance) and bearing durability.  
     In the lubricating oil for a rolling bearing used in a high-speed rotating equipment, the oil is made of nondrying vegetable oil (rice bran oil) extracted from rice bran.

FIELD OF INVENTION

[0001] The present invention relates to a lubricating oil for a rollingbearing used in a high-speed rotating equipment.

[0002] More specifically, the present invention relates to a novel andhigh-performance lubricating oil used for a rolling bearing that is amajor component of a high-speed rotating equipment (an air turbine handpiece) used in, among others, the medical and food-processing fields.

[0003] Further more specifically, the present invention relates to anovel and high-performance lubricating oil that is excellent inbiosafety (reduced harm to human body), environmental conservation(safety to the environment) and bearing durability and is to be appliedto a rolling bearing used in a high-speed cutter (an air turbine handpiece).

BACKGROUND ART

[0004] A high-speed cutter is generally constructed of a rotary shaftfor fixedly holding various cutting tools thereon, a drive unit forrotating the rotary shaft, and a bearing unit rotatably supporting therotary shaft.

[0005] As a high-speed cutter of this type, an odontotherapeutichigh-speed cutter (i.e., an air turbine hand piece) can be mentioned byway of example.

[0006] Bearing units for the above-described odontotherapeutichigh-speed cutters (i.e., an air turbine hand pieces) are known toinclude those having a ball bearing system making use of balls (rollingelements) and those equipped with a (non-contact) air bearing systemmaking use of an air bearing.

[0007] Paying attention, for example, to the bearing mechanisms ofdental air turbine hand pieces, two types of air turbine hand pieces areknown, one being of the ball bearing turbine type and the other of theair bearing turbine type.

[0008] The former type, i.e., the ball bearing turbine type can beconsidered to be a high-speed rotation type for approximately 200,000 to400,000 rpm, while the latter type, that is, the air bearing turbinetype can be considered to be a super high-speed rotation type forapproximately 300,000 to 500,000 rpm.

[0009] It is, however, to be noted that the above-described revolutionspeeds of the ball bearing turbine type and the air bearing turbine typeare general values. For example, though the dental air turbine handpiece already proposed by the inventors of the present application(Japanese Patent No. 3,122,302 and U.S. Pat. No. 5,562,446) is of theball bearing turbine type, it has high performance so that superhigh-speed rotation can be achieved.

[0010] To facilitate the understanding of the conventional art and thepresent invention, a description will now be made about the constructionof an equipment to which the lubricating oil according to the presentinvention is applied, namely, of a dental high-speed cutter (i.e., adental air turbine hand piece).

[0011]FIG. 1 to FIG. 2 show the construction of a dental air turbinehand piece. FIG. 1 is a perspective view illustrating the overallconstruction, and FIG. 2 is a cross-sectional view illustrating theinternal construction of the head portion and the neck portion inparticular.

[0012] As is depicted in FIG. 1, the dental air turbine hand piecedesignated generally by letter A is composed of a head portion H, whichcarries a cutting tool B(5) fixedly held on a rotor shaft (drive shaft)of an air turbine, and a grip portion G.

[0013] The neck portion N of the grip portion G is connected to the headH and is internally provided with means for supplying compressed air tothe air turbine arranged within the head portion H and also fordischarging compressed air from the air turbine.

[0014]FIG. 2 illustrates the internal construction of the head portion Hand the neck portion N of the dental air turbine hand piece A.

[0015] As is illustrated in FIGS. 1 and 2, in the head portion H, aturbine rotor shaft 3 with turbine blades 2 disposed at a peripheraledge portion thereof is arranged within a chamber 11 of a head 1, andthe turbine rotor shaft 3 is rotatably supported within the head 1 byway of a bearing unit 4.

[0016] The head 1 is composed of a head main part 12 and a cap part 13.Within the head main part 12, the bearing unit 4 is arranged torotatably support the turbine rotor shaft 3. The cutting tool 5 thatperforms a dental treatment is fixedly held in a bore which is formedthrough the turbine rotor shaft 3 along its central axis. Incidentally,the cutting tool 5 is provided on its peripheral side wall with a chuck51 for holding the cutting tool 5 in place within the bore.

[0017] The bearing unit 4 is of a ball bearing type and is constructedof an inner ring 41, an outer ring 42, rolling elements 43 and aretainer 44. The bearing unit 4 can be provided on an outer periphery orside wall thereof with O-rings for providing the bearing unit withself-centering function and/or with known wave washers for enhancing therigidity of the shaft.

[0018] A main part 6 of the neck portion N is provided with an airsupply passage 7 and an air inlet 71 for supplying compressed air to theturbine blades 2 arranged within the chamber 11 and also with airdischarge passages 8,9 and air outlets 81,91 for discharging compressedair from the chamber 11.

[0019] In the above-described internal construction of the dental airturbine hand piece A as illustrated in FIG. 2, the means for supplyingand discharging compressed air is the one proposed by the inventors ofthe present application (Japanese Patent No. 3,122,302 and U.S. Pat. No.5,562,446) and is of a new construction absolutely unseen in theconventional art.

[0020] Accordingly, FIG. 2 contains other reference symbols in additionto those referred to in the above to describe the individual elements(members). Although a description of these additional symbols is omittedherein, the construction of the conventional dental air turbine handpiece can be easily understood on the basis of FIG. 2.

[0021] As described above, the dental air turbine hand piece A—which isequipped with the air supply and discharge means shown in FIG. 2 andalready proposed by the inventors of the present application—belongs tothe category of conventional hand pieces with a rolling bearing builttherein, but it makes it possible to obtain rotation of an extremelyhigh speed and hence a large torque.

[0022] In the above-described dental air turbine hand piece of the ballbearing type, its bearing unit is in the form of a miniature bearingunit. Since the turbine rotor shaft rotates at a high speed ofapproximately 200,000 to 400,000 revolutions per minute, the temperatureinside the bearing unit becomes high, and the bearing unit is exposed toa large stress. For a lubricating oil to be applied to a bearing whichis used under the above-described severe conditions, it is, therefore,extremely important to control its quality and properties.

[0023] Further, the above-described dental air turbine hand piece of theball bearing type is used in the oral cavity. Accordingly, the dentalair turbine hand piece is used by spraying or dropping a lubricating oilto the bearing unit; in other words, the turbine is used under anenvironment of minimum lubrication, and is subjected to high-pressureand high-temperature treatment for sterilization and disinfection (whichis also called “autoclaving”, which is applied under the followingconditions: steam pressure, 2.4 kgf/cm²; temperature, 135° C.; time,five minutes).

[0024] The lubricating oil for use in the above bearing unit is,therefore, required to have properties sufficient to withstand suchsevere use conditions, such as oxidation resistance.

[0025] As lubricating oils for rolling bearing units in high-speedcutters such as a dental air turbine hand piece, a variety oflubricating oils have been used or proposed to date.

[0026] For example, it is widely practiced to supply a lubricating oilby spraying it with flon or LPG. As lubricating oils, those making useof refined mineral oils, such as paraffin, as base oils are known.

[0027] The above-described lubricating oils are typically of thepetroleum base and are prepared by fractionating and refining petroleuminto various fractions and incorporating additives, such as an oxidationinhibitor, therein as needed.

[0028] As base oil components of the above-described lubricating oils,there are known synthetic oils such as glycol esters and low molecularweight polyolefins in addition to natural mineral oils.

[0029] Furthermore, edible oils such as animal oils and vegetable oilsare also known to be in use as lubrication oils for precision machines,machine tools, ship engines and the like. In general, however, theseedible oils are used by being added in proportions of from 10 to 20 wt.% to lubricating oil of the mineral oil base.

[0030] Incidentally, the edible oils have a problem in oxidationresistance. It is, therefore, a common practice to use such edible oilsin combination with one or more of various oxidation stabilizers(oxidation inhibitors).

[0031] In view of the above-described severe use conditions for dentalair turbine hand pieces, dental air turbine hand pieces equipped withretainers impregnated with fluorinated oils, which have excellent heatresistance, permit sterilization and disinfection (autoclaving) and havesuperb lubricity, have been proposed recently in Japanese PatentApplication Publication (Kokoku) No. HEI 5-43884 and Japanese UtilityModel Application Laid-Open (Kokai) No. HEI 7-10553.

[0032] Incidentally, the retainers are in the form of porous membersobtained by sintering powder of a polyimide resin. The fluorinated oilshave properties such that they are inactive, are excellent in heatresistance, chemical resistance and solvent resistance and, even whenexposed to high temperatures, do not form a solid deteriorationmaterial. It is, therefore, possible to consider that theabove-described proposed dental air turbine hand pieces use theseproperties for the retainers.

[0033] Further, Japanese Patent Application Laid-Open (Kokai) No. HEI6-165790 discloses an embodiment of impregnating with a lubricating oila snap retainer of a ball bearing in a dental air turbine hand piece tobe described briefly hereinafter, although its direct subject matter isthe snap retainer itself.

[0034] More specifically, in the dental air turbine hand piece disclosedin Japanese Patent Application Laid-Open (Kokai) No. HEI 6-165790referred to in the above, the snap retainer of the ball bearing ischaracterized in that:

[0035] (i) the retainer is a snap retainer, in which a ball-holdingpocket is formed on one side of a synthetic resin cylinder having afabric-made fiber layer therein, and chamfered portions are formed onopening-side end edges of the pocket; and

[0036] (ii) the fiber layer of the retainer is impregnated with alubricating oil.

[0037] The snap retainer of the ball bearing disclosed in JapanesePatent Application Laid-Open (Kokai) No. HEI 6-165790 is to improve thebalance of rotation by the feature (i) above, thereby making it possibleto protect the retainer from wearing and also to avoid an increase inrotating torque, both of which would otherwise occur due to contactbetween the retainer and the outer ring.

[0038] However, the specific composition of the lubricating oilimpregnated in the retainer is not clear in Japanese Patent ApplicationLaid-Open (Kokai) No. HEI 6-165790. As a matter of fact, Japanese PatentApplication Laid-Open (Kokai) No. HEI 6-165790 does not disclosewhatsoever the specific composition of the edible oil, although itdescribes an embodiment of charging an edible oil into a housing in thedescription of the conventional art. It is the belief of the inventorsof the present application that the lubricating oil or edible oildisclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI6-165790 is still no better than the level of theconventionally-proposed lubrication oils in view of the technical levelin the present field of art.

[0039] Further, Japanese Patent Application Laid-Open (Kokai) No. HEI6-212179 discloses mixing of ceramic powder in a lubricating oil toreduce the replenishing frequency of a lubricating oil to a bearing of adental air turbine. This is a proposal based on a finding that advancemixing of ceramic powder in a lubricating oil can improve the lifetimeof the lubricating oil.

[0040] The above-described various lubricating oils, which have beenproposed to date, are still accompanied by various drawbacks whichshould be improved as described below in order to permit theirapplication in high-speed rotating equipments such as anodontotherapeutic air turbine hand piece having a rolling bearing.

[0041] For example, the above-described lubricating oils of the mineraloil base, such as of the liquid paraffin base, or of the synthetic oilbase or the lubricating oils obtained by adding edible oils to themstill involve matters to be improved from the viewpoint of biosafety andenvironmental conservation.

[0042] In addition, the fluorinated oils proposed in the above-describedJapanese Patent Application Publication (Kokoku) No. HEI 5-43884 andJapanese Utility Model Application Laid-Open (Kokai) No. HEI 7-10553,such as perfluoropolyether (PFPE) and perfluoropolyalkyl ether (PFAE),are excellent in heat resistance, chemical resistance and solventresistance; and even when exposed to high temperatures, they form nosolid deterioration material. They are, accordingly, preferred aslubricating oils for a high-speed rotating equipments such as an airturbine hand piece. However, the fluorinated oils still involve mattersto be improved from the viewpoint of environmental destruction andbiosafety.

[0043] Furthermore, the above-described Japanese Patent ApplicationLaid-Open (Kokai) No. HEI 6-165790 discloses impregnation of a retainerof a rolling bearing, which is formed of a shaped member made of aphenol resin and having a fiber layer, with a lubricating oil; and itsuggests use of an edible oil as a lubricating oil. As will be descriedin detail subsequently herein, a great majority of general edible oilsare drying oils; and when they become dry, they are formed into resinoussolids. They are, accordingly, not suited as durable lubricating oilsfor bearings.

[0044] As described above, most of general edible oils are dry oils andare susceptible to oxidation. It is, therefore, a common practice to usethem by adding a synthetic oxidation inhibitor therein. In anedible-oil-base lubricating oil composed primarily of drying oils andadded with an oxidation inhibitor, it is necessary to keep in mind thepotential risk that a reaction product to be formed between theoxidation inhibitor and a metal dissolved out from a bearing system maybe a material harmful for the body.

[0045] According to the above-described Japanese Patent ApplicationLaid-Open (Kokai) No. HEI 6-212179, ceramic powder is mixed in alubricating oil to improve the lifetime of the lubricating oil so thatthe replenishing frequency of the lubricating oil to a lubricationsystem can be reduced. In an odontotherapeutic cutter of the high-speedrotation type (an air turbine hand piece), however, the race (retainer)and balls of the bearing are ground by the ceramic powder so that ametal harmful for the body may be dissolved out, or a serious damage canbe given to the bearing mechanism when the lubricating oil becomesscarce under the environment of minimum lubrication.

[0046] As described above, the conventional lubricating oils for rollingbearings, said oils being for use in a high-speed rotating equipments,for example, an odontotherapeutic high-speed cutter (an air turbine handpiece), still involve drawbacks when evaluated in view of the followingproperties:

[0047] (i) biosafety (reduced harm to the body),

[0048] (ii) environmental conservation (safety),

[0049] (iii) heat resistance (the possibility of sterilization anddisinfection by autoclaving), and

[0050] (iv) durable lubrication system.

DISCLOSURE OF INVENTION

[0051] The present invention is completed in view of the above-describedproblems of the conventional art.

[0052] Incidentally, the direct motive for the present invention residedin the lack of a lubricating oil of excellent properties for ahigh-performance and high-speed rotating dental air turbine hand pieceof a ball bearing turbine type previously proposed by the inventors ofthe present application (Japanese Patent No. 3,122,302 and U.S. Pat. No.5,562,446).

[0053] The present invention provides a novel lubricating oil that hasexcellent properties in connection with the above-described evaluationcharacteristics such as biosafety.

[0054] More specifically, the present invention provides a novellubricating oil capable of meeting the requirements that in a high-speedrotating equipment such as an odontotherapeutic high-speed cutter (aball bearing type air turbine hand piece), it can be used under largetorques produced by high-speed rotation, it has an excellent heatresistance, in other words, it can permit sterilization and disinfectionby autoclaving under high temperature and high pressure, and it can beused stably over a long period of time.

[0055] In other words, the present invention provides a novellubricating oil that is for a rolling bearing used as a principalelement in a high-speed rotating equipment of a ball bearing typeoperated at 200,000 rpm or higher or even at 300,000 rpm or higher suchas an odontotherapeutic high-speed cutter and is capable of establishinga durable lubrication system without needing a reduction in revolutionspeed in a high-speed rotating equipment.

[0056] The inventors of the present application conducted diligentresearches in an attempt to provide a lubricating oil that has theabove-described high performance. As a result of the researches, theinventors previously proposed a lubricating oil composed of a specificnondrying vegetable oil for use in the ball bearing of a high-speedrotating equipment (Japanese Patent No. 3,195,545 and U.S. Pat. No.5,981,448).

[0057] The lubricating oil of the above-described nondrying vegetableoil previously proposed by the inventors is comprised of:

[0058] (i) 60 wt. % or greater of at least one type of monovalentunsaturated fatty acid that contains one unsaturated bond per molecule,and

[0059] (ii) 30 wt. % or less of a polyvalent unsaturated fatty acid thatcontains at least two unsaturated bonds per molecule.

[0060] More specifically, the nondrying vegetable oil previouslyproposed by the inventors of the present application is constructed fromat least one component selected from the group consisting of:

[0061] olive oil,

[0062] arachis oil, and

[0063] oleysol oil.

[0064] The inventor of the present application subsequently conductedfurther diligent researches on vegetable oil type lubricating oils thathave a higher performance surpassing that of the previously proposedlubricating oil.

[0065] As a result, the inventor of the present application discoveredthat a nondrying vegetable oil obtained from rice bran has a highperformance as a lubricating oil used in the bearing of a high-speedrotating equipment (an air turbine hand piece). Such a nondryingvegetable oil obtained from rice bran is a similar nondrying vegetableoil type lubricating oil to the previously proposed oil but differstherefrom; and when focusing on the constituent components of glycerolesters, in this oil from rice bran:

[0066] the content of monovalent unsaturated fatty acids (in concreteterms, oleic acid) that are relatively stable against oxidation is lowerthan that of the previously proposed oil, and

[0067] the content of divalent unsaturated fatty acids (in concreteterms, linolic acid) that are relatively unstable with respect tooxidation is higher than that of the previously proposed oil.

[0068] The invention of the present application was made based upon theabove-described findings.

[0069] The present invention provides a rice bran type lubricating oilfor a ball bearing of a high-speed rotating equipment which is superiorin terms of biosafety and environmental protection, which is superior interms of heat resistance and durability, and which originates in anatural vegetable that is superior in terms of economy.

[0070] In summary, the present invention relates is a lubricating oilfor a rolling bearing of a high-speed rotating equipment, and thepresent invention is characterized in that the lubricating oil comprisesa nondrying vegetable oil extracted from rice bran.

[0071] More specifically, the present invention relates to a lubricatingoil for a rolling bearing of a high-speed rotating equipment which ischaracterized in that the oil is a nondrying vegetable oil extractedfrom rice bran and is a rice bran type nondrying vegetable oil thatcontains:

[0072] oleic acid (monovalent unsaturated fatty acid)

[0073] 40-45 wt. %

[0074] linolic acid (divalent unsaturated fatty acid)

[0075] 35-40 wt. %

[0076] palmitic acid (C₁₋₆ saturated fatty acid)

[0077] 15-20 wt. %

[0078] other

[0079] balance (100 wt. %)

BRIEF DESCRIPTION OF DRAWINGS

[0080]FIG. 1 is a perspective view of a dental air turbine hand piece ofa high-speed rotation type to which a lubricating oil according to thepresent invention is applied; and

[0081]FIG. 2 is a cross-sectional view of a head portion (H) and a neckportion (N) of the dental air turbine hand piece of FIG. 1, FIG. 2showing the structures of the head portion (H) and neck portion (N).

BEST MODE FOR CARRYING OUT THE INVENTION

[0082] As described above, the characteristic feature of the presentinvention resides in the novel constitution of a lubricating oil appliedto a rolling bearing of a high-speed rotating equipment, for example, anodontotherapeutic high-speed cutter (an air turbine hand piece), havingas an essential element a rolling bearing rotatably supporting a rotaryshaft thereon.

[0083] More specifically, the characteristic feature of the presentinvention resides in that, in a high-speed rotating equipment, forexample, a high-speed cutter (an air turbine hand piece), having arolling bearing element composed of an outer ring, an inner ring,rolling elements (ball bearings) and a retainer made of a metal or aheat-resistant synthetic resin and rotatably supporting a rotary shaftwith air turbine blades fixed thereon, a lubricating oil applied to therolling bearing element is composed of a nondrying oil made of vegetableoils excellent in biosafety and environmental conservation and superb inheat resistance (so that sterilization and disinfection by autoclavingis feasible) and durability in place of a conventional lubricating oilof the mineral oil base or of the synthetic oil base.

[0084] The technical structure and embodiments of the present inventionwill be described below in detail.

[0085] First, a description will be made in detail about thecharacteristic feature of the present invention in which the lubricatingoil is composed of a vegetable oil, especially, a specific nondryingvegetable oil.

[0086] To faciliate the understanding of the present invention,vegetable oils will be described below in general.

[0087] Roughly dividing, vegetable oils can be classified into thefollowing three types:

[0088] (i) Nondrying Oils:

[0089] The term “nondrying oil” means an oil which does not form anyfilm-like matter (resinous solid) even when dried (oxidized) in the formof a thin layer in air.

[0090] The nondrying oil of this type contains unsaturated fatty acids,each of which contains two or more double bonds per molecule(hereinafter called “poly-unsaturated fatty acids), only in smallamounts and is primarily composed of the glyceride (glycerol ester) ofoleic acid (which contains one double bond per molecule), and its iodinevalue (a scale indicating the degree of unsaturation of an oil) is 100or smaller.

[0091] Representative examples of nondrying oils of this type include,among others, olive oil, arachis oil and oleysol oil.

[0092] As described above, the present invention relates to a rice brantype nondrying oil. Such oil has not attracted much attention in thepast, and use only as edible oils has been recognized.

[0093] As described above, the inventor of the present application hassecured Japanese Patent No. 3,195,545 and U.S. Pat. No. 5,981,448 inwhich functions and advantages far superior to those of conventionallubricating oils are obtained when nondrying vegetable oils such asolive oil, arachis oil and oleysol oil, etc. as described above are usedas lubricating oils in a bearing of a high-speed rotating equipment.

[0094] The rice bran type nondrying oil that is the principal componentof the lubricating oil of the present invention, as will be described indetail later, belongs to the category of a nondrying vegetable oil, andit differs from the oil previously proposed by the inventor.

[0095] (ii) Semidrying Oils:

[0096] The “semidrying oil” is an oil which shows propertiesintermediate between the nondrying oil described above and the dryingoil described below.

[0097] The iodine value of the semidrying oil is from 100 to 130.

[0098] Representative examples of the semidrying oils of this typeinclude, among others, rapeseed oil, sesame oil and cotton seed oil.

[0099] (iii) Drying Oils:

[0100] The term “drying oil” means an oil which forms a film (resinoussolid) when dried (oxidized) in the form of a thin layer in air.

[0101] The drying oil of this type is composed of glycerides of fattyacids having high degrees of unsaturation (for example, linoleic acidcontains two double bonds and linolenic contains three double bonds).These glycerides absorb oxygen in the air and undergo oxidativepolymerization, thus easily forming a film-like matter. The iodine valueof such a drying oil is 130 or greater.

[0102] Representative examples of drying oils of this type includelinseed oil and tung oil.

[0103] Among the above-described various types of vegetable oils, thenondrying oils are oils and fats (glycerol esters of fatty acids) eachof which does not make a film-like matter (resinous solid) even when itis formed into a thin layer and dried (oxidized). Since they areexcellent in heat resistance (so that sterilization and disinfection byautoclaving is feasible) and durability, they are suitable aslubricating oils for a bearing used in a high-speed rotating equipment(an air turbine hand piece).

[0104] The present invention adopts, as a lubricating oil for a bearingin a high-speed rotating equipment (an air turbine hand piece), a ricebran oil that is extracted, as a nondrying vegetable oil, from ricebran.

[0105] The rice bran type nondrying vegetable oil according to thepresent invention will be described below in detail in comparison witholive oil, which is a typical example of the nondrying vegetable oil theinventor of the present application has previously proposed (seeJapanese Patent No. 3,195,545 and U.S. Pat. No. 5,981,448).

[0106] Olive oil will be described first, and then the rice bran oil ofthe present invention will be described in comparison with the oliveoil.

[0107] Olive oil is an oil (glycerol esters) available from drupes ofOlea Europaea. Roughly driving, its components can be classified intothe following three types:

[0108] (i) unsaturated resin acids;

[0109] (ii) saturated resin acids; and

[0110] (iii) various trace components.

[0111] The unsaturated resin acids in olive oil are generally composedof mono-unsaturated and di- and higher-unsaturated (poly-unsaturated)resin acids.

[0112] The kinds and contents of the unsaturated resin acids in oliveoil will be shown below: 1) Oleic acid (mono-unsaturated) 56.0-83.0%  CH₃(CH₂)₇CH═CH(CH₂)₇COOH 2) Linoleic acid (poly-unsaturated) 3.5-20.0%   CH₃(CH₂)₄CH═CHCH₂CH═CH(CH₂)₇COOH 3) Palmitoleic acid(mono-unsaturated) 0.3-3.5%   CH₃(CH₂)₅CH═CH(CH₂)₇COOH 4) Linolenic acid(poly-unsaturated) 0.0-1.5%   CH₃CH₂CH═CHCH₂CH═CHCH₂CH═   CH(CH₂)₇COOH5) Gadoleic acid (mono-unsaturated)  0.0-0.05%  CH₃(CH₂)₉CH═CH(CH₂)₇COOH

[0113] As described above, olive oil abundantly contains oleic acidwhich is a mono-unsaturated fatty acid. Olive oil also containspoly-unsaturated fatty acids, such as linoleic acid, in small amounts.

[0114] As has also been described above, a poly-unsaturated fatty acidis susceptible to oxidation. Nonetheless, olive oil has excellentoxidation resistance as a whole because, as will be describedsubsequently herein, olive oil contains tocopherols (vitamin E) as tracecomponents and poly-unsaturated fatty acids such as linolenic acid areprotected from oxidative deterioration owing to the anti-oxidationaction of the tocopherols (vitamin E).

[0115] A description will next be made about saturated fatty acidcomponents in olive oil.

[0116] The kinds and contents of saturated fatty acids in olive oil willbe shown below: 1) Palmitic acid CH₃(CH₂)₁₄COOH  7.5-20.0% 2) Stearicacid CH₃(CH₂)₁₆COOH 0.5-3.5% 3) Myristic acid CH₃(CH₂)₁₂COOH  0.0-0.05%4) Arachidic acid CH₃(CH₂)₁₈COOH  0.0-0.05% 5) Behenic acidCH₃(CH₂)₂₀COOH  0.0-0.05% 6) Lignoceric acid CH₃(CH₂)₂₂COOH  0.0-0.05%

[0117] As is recognized from the foregoing, olive oil can be consideredto have low contents of saturated fatty acids which causehypercholesterolemia.

[0118] Next, a description will be made on various trace components inolive oil.

[0119] The kinds of various trace components in olive oil willhereinafter be described along with their properties and functions.

[0120] (1) Unsaponifiable Materials:

[0121] (a) Sterols

[0122] (b) Hydrocarbons

[0123] Squalene

[0124] Aromatic hydrocarbons (which impart inherent sensorycharacteristics, namely, aroma and flavor)

[0125] (c) Tocopherols (oxidation-preventing function)

[0126] α-tocopherol (vitamin E) (prevention of blackening andpolymerization)

[0127] β-tocopherols (prevention of rancidity which would otherwise becaused by the existence of one or more heavy metals)

[0128] Incidentally, olive oil and other nondrying oils such as arachisoil and oleysol oil contain α-tocophenal (vitamin E) as mach as 150 wtppm (150 mg/kg).

[0129] (d) Triterpene alcohols

[0130] Cycloalterenol

[0131] Erythrodiol

[0132] (e) Fat-soluble vitamins

[0133] Vitamins A, D (anti-oxidation effects)

[0134] (2) Phospholipids, Chlorophyll and Derivatives:

[0135] (a) Phospholipids

[0136] (b) Chlorophyll (anti-oxidation effect)

[0137] (3) Phenolic Compounds:

[0138] (a) Phenolic compounds (anti-oxidation effects)

[0139] (b) Polyphenols (anti-oxidation effects)

[0140] As described above, olive oil has higher contents of varioustrace components, which act against oxidation of oils and fats, thanother nondrying oils and drying oils; and it can, therefore, provide alubricating oil excellent in heat resistance (so that sterilization anddisinfection by autoclaving is feasible) and durability.

[0141] The inventor of the present application discovered that thelubricating characteristics improve as the total content of free fattyacids (saturated or unsaturated) contained in nondrying vegetable oilsdecreases.

[0142] The free fatty acids contained in nondrying vegetable oils(occasionally called “free fatty acids” in the following description)will be described.

[0143] Generally, oils and fats (fats such as beef tallow, lard andbutter, etc. and fatty oils such as rapeseed oil, tung oil and linseedoil, etc) are composed of glycerol esters of higher fatty acids.

[0144] More specifically, various types of fatty acids (saturated orunsaturated) are present in nondrying vegetable oils as esters shown bythe following formula (1)

3 molecules of fatty acid+1 molecule of glycerol→1 molecule oftriglyceride (ester)  (1)

[0145] However, the above-described nondrying vegetable oils includevarious types of fatty acids (free fatty acids) that do not bond withglycerol (CH₂OH—CHOH—CH₂OH). If the total content of the above-describedfree fatty acids is indicated by free acidity, then the acidity drops asthe free acidity value drops, and the viscosity also shifts toward ahigher viscosity level. Accordingly, nondrying vegetable oils with a lowfree acidity are superior in terms of durability as lubricating oils foruse in a rolling bearing.

[0146] If the quality of “Olive Oil” (trade name of olive oilmanufactured by Golden Eagle Olive Products, U.S.A.) is classified basedupon the free acidity, the following results are obtained: 1). Freeacidity of refined oil 0.1 wt. % 2). Free acidity of extra-virgin oliveoil 1.1 wt. % 3). Free acidity of virgin olive oil 3.2 wt. %

[0147] The free acidity of nondrying vegetable oils such as olive oil,etc. can be lowered by a method in which, for instance, a 5 to 10%aqueous solution of sodium hydroxide is added to olive oil and heated,the olive oil is saponified so as to produce glycerol and sodium saltsof fatty acids, and the glycerol thus produced forms ester bonds withfree fatty acids, and then the oil and fat components are separated andremoved by centrifugal separation, etc. The resulting olive oil is lowin free acidity.

[0148] Next, the rice bran type nondrying vegetable oil (called “ricebran oil”) of the present invention will be described. The oil has theproperties and characteristics described below.

[0149] In the present invention, various types of rice bran oils can beused. The “Komeabura” (pure rice oil) rice bran oil manufactured byTsuno Food Industries Co., Ltd. is one of them.

[0150] In regards to rice bran oils, it can be remembered that a majoroil-related syndrome incident occurred in 1968 in Japan when a heattransfer medium (PCB) was mixed with rice bran oil in the finaldeodorization process.

[0151] Subsequently, however, the deodorization process was changed to asteam distillation system in which a high-pressure boiler is used, andno heat transfer medium is employed. Safety is thus now ensured.

[0152] The most common method form manufacturing rice bran oil includesthe following steps: original rice oil→degumming step→dewaxingstep→deacidifying step→decoloring step→deodorization step.

[0153] 1). Qualitative and Quantitative Analysis Results for Fatty Acidsin Rice Bran Oil:

[0154] A qualitative analysis of rice bran oil was performed byhydrolyzing the rice bran oil, converting the oil into a methyl esterwith trimethylsilyldiazomethan, and then performing an analysis using agas chromatography/mass spectrometry (GC/MS).

[0155] A quantative analysis of rice bran oil was performed by gaschromatography (GC). The results obtained (contents, shown by wt. %) areshown below: Oleic acid (monovalent unsaturated) 41.0 Linolic acid(divalent unsaturated) 36.6 Eicosenic acid (monovalent unsaturated) 0.7Myristic acid (C₁₃ saturated) 0.3 Palmitic acid (C₁₅ saturated) 15.9Stearic acid (C₁₇ saturated) 1.8 Arachidonic acid (C₁₉ saturated) 0.7Other 3.0

[0156] Compared to olive oil, rice bran oil has the following specialfeatures:

[0157] The content of oleic acid which is a monovalent unsaturated acidthat is relatively stable against oxidation is small; and

[0158] The content of linolic acid which is a divalent unsaturated acidthat is relatively unsatable with respect to oxidation is large.

[0159] When rice bran oil is viewed only based upon these points, ricebran oil might appear to have poor lubricating characteristics comparedto olive oil. However, as will be seen below, rice bran oil showssuperior lubricating characteristics because of other componentscontained in rice bran oil

[0160] 2) Mass Analysis of Free Oleic Acid/Linolic Acid:

[0161] After being dissolved in THF (tetrahydrofuran), rice bran oil wasanalyzed by high-performance liquid chromatography (HPLC). The resultsobtained (contents following liberation, shown by wt. %) Free oleic acidless than 0.01 Free linolic acid less than 0.01

[0162] Thus, it is recognized that the free fatty acid content of ricebran oil is less than {fraction (1/10)} that of the highest grade ofolive oil (refined oil). It is inferred that this characteristiccontributes greatly to the oxidation resistance, etc.

[0163] 3). Quantative Analysis of α-Tocophenol (Vitamin E)

[0164] Following dissolution in an eluent solution (n-Hx/2POH=99/1),analysis was performed by high-performance liquid chromatography (HPLC).A quantative analysis was performed also for δ-tocophenol. The resultsobtained (contents, shown by wt.ppm) are shown below. α-tocophenol 600δ-tocophenol less than 10

[0165] It is recognized that the α-tocophenol (vitamin E) content ofrice bran oil is far higher than that of olive oil (the α-tocophenolcontent of olive oil is approximately 150 wt.ppm). It is inferred thatthis characteristic contributes greatly to the oxidation resistance,etc.

[0166] Differences between rice bran oil, which belongs to nondryingvegetable oils and makes up the lubricating oil of the present inventionfor a rolling bearing for a high-speed rotating equipment (a dental airturbine hand piece), and other edible oils, which include nondryingvegetable oils such as olive oil, are shown below in Table 1.

[0167] In Table 1, the notes have the following meanings:

[0168] (1) composed primarily of oleic acid, and containing palmitoleicacid,

[0169] (2) linoleic acid,

[0170] (3) linolenic acid, and

[0171] (4) composed of palmitic acid, stearic acid, lauric acid andmyristic acid.

[0172] In Table 1, asterisk (*) indicates vegetable oils for comparison.TABLE 1 Comparison between Nondrying Vegetable Oils and other EdibleOils Average constituent fatty acids (%) Unsaturated fatty acidsSaturated fatty Mono- Di- Tri- acids Vitamin E (1) (2) (3) (4) (mg/kg)Rice bran oil 40˜45 35˜40 — 15˜20 600 Olive oil* 70 12 0.5-1.5 15 150Arachis oil* 60 25 — 15 150 Oleysol oil* 80 10 — 10 — Rapeseed oil* 6022 10  8 150 Sunfrower oil* 20 70 10 10 250 Soybean oil* 23 55 7 15 175Corn oil* 26 60 2 12 200 Coconut oil*  8  2 <1 90 — (solid fat)

[0173] In the lubricating oil for use in the rolling bearing of ahigh-speed rotating equipment (an air turbine hand piece) provided bythe present invention, the rice bran oil, which constitutes the base oiland belongs to the category of nondrying vegetable oil, can be used asan independent (sole) base oil, and it can be blended with othernondrying vegetable oils (such as olive oil, arachis oil, oleysol oil,etc.) besides rice bran oil so as to form a blended type lubricatingoil.

[0174] Furthermore, oil-absorbing synthetic resin particles can be addedto the rice bran oil as an additive component.

[0175] The inventor of the present invention have found that particlesof an oil-absorbing synthetic resin (oil-absorbing crosslinked polymer)such as a crosslinked polymer of the acrylate ester type are extremelyeffective for improving the retention of the lubricating oil for arolling bearing without impairing the properties of the rice bran oil.

[0176] A description will be made below on the oil-absorbing crosslinkedpolymer which is preferred for improving the properties of rice bran oiluseful in the practice of the present invention.

[0177] Incidentally, the oil-absorbing crosslinked polymer itself isknown in the present field of art from Japanese Patent ApplicationLaid-Open (Kokai) No. HEI 5-337367 and Japanese Patent ApplicationPublication (Kokoku) No. HEI 3-143996.

[0178] However, needless to say, it is found for the first time by theinventor of the present application that oil-absorbing crosslinkedpolymer imparts excellent lubrication characteristics for the rice brantype lubricating oil used in a rolling bearing of a high-speed rotationsystem.

[0179] In the present invention, when oil-absorbing crosslinked polymeris co-used, several times more of excellent lubrication characteristicscan be obtained compared to cases in which such oil-absorbingcrosslinked polymer is not co-used.

[0180] The solubility parameter (SP value) of rice bran oil used as aprincipal component of the lubricating oil of the present invention isbetween 6 and 9. It is, therefore, preferable that the oil-absorbingcrosslinked polymer used in the present invention have a similar SPvalue from the viewpoint of compatibility. In other words, it ispreferable that an oil-absorbing crosslinked polymer of 9 or smaller inSP value be added to the lubricating oil of the present invention whichcontains rice bran oil as its principal component.

[0181] The above-described oil-absorbing crosslinked polymer can beproduced generally by copolymerization of:

[0182] (i) 90 to 99.9 wt. % of a monomer (A) containing onepolymerizable unsaturated group per molecule and capable of producing apolymer having an SP value of 9 or smaller, and

[0183] (ii) 0.1 to 10 wt. % of a crosslinking monomer (B) containing atleast two polymerizable unsaturated group per molecule.

[0184] The monomer (A) above comprises at least one polymerizableunsaturated monomer containing:

[0185] (1) at least one C₂-C₃₀ aliphatic hydrocarbon group; and selectedfrom the group consisting of:

[0186] (2) alkyl(meth)acrylate, alkylaryl(meth)acrylate,alkyl(meth)acrylamide, alkylaryl(meth)acrylamide,alkylaryl(meth)acrylamdie, fatty acid vinyl esters, alkylstyrenes, andα-olefins.

[0187] The crosslinking monomer (B) above comprises, for instance:

[0188] ethylene glycol di(meth)acrylate, diethylene glycoldi(meth)arylagte, polyethylene glycol di(meth)acrylate, propyleneglycoldi(meth)acrylate, polypropylene glycol di(meth)acrylate, 1,3-butyleneglycol di(meth)acrylate, neopentyl glycol di(meth)acrylate,N,N′-methylene bisacrylamide, N,N′-propylenebisacrylamide, glyceroltri(meth)acrylate, trimethylolpropane tri(meth)-acrylate, anddivinylbenzene.

[0189] The above-described oil-absorbing crosslinking polymer can alsobe produced by using a monomer containing two polymerizable unsaturatedgroups per molecule, specifically a diene monomer. Illustrative examplesof oil-absorbing crosslinked polymers making use of diene monomers ofthis type can include polymers of butadiene, isoprene, cyclopentadieneand 1,3-pentadiene, and their hydrogenation products; and copolymers ofthe above-described dienes with other polymerizable monomers such asstyrene and α-olefins such as butylene, and their hydrogenationproducts.

[0190] Incidentally, the above-described crosslinking monomers can beused as the polymerizable monomers.

[0191] Further, the oil-absorbing crosslinked polymer may be composed ofa crosslinked copolymer of ethylene with another olefin. Examples of theanother olefin copolymerize with ethylene can include propylene,butylene and pentene. Further, as crosslinking monomers, those mentionedabove can be used.

[0192] The above-described oil-absorbing crosslinked polymer is in theform of particles with an average particle size from 0.5 to 2,000 μm,and it can be added in a desired amount to the rice bran oil. As anoil-absorbing crosslinked polymer of this type, it is possible to use“Oleosorb PW-190” (trade name) or “Oleosorb PW-170” (trade name), bothacrylate ester polymers produced by Nippon Shokubai Co., Ltd.

[0193] Next, a description will be made about a bearing, particularly aretainer (a retaining element used in a bearing), to which theabove-described lubricating oil according to the present invention thatis composed primarily of the rice bran oil, is applied.

[0194] In the present invention, the retainer can be those made ofphenol resin or polyimide resin. A polyimide resin retainer will bedescribed below.

[0195] The polyimide resin (hereinafter abbreviated as “PI resin”) usedin the present invention is a resin obtained by subjecting an aromaticcarboxylic acid and an aromatic amine to condensation polymerization andcontaining imide bonds in the backbone thereof (which may be eitherthermoplastic or thermosetting), and it is excellent in heat resistance,chemical resistance, mechanical properties and electricalcharacteristics.

[0196] In the present invention, the term “PI resin” should beinterpreted to also include polyamideimide resins (hereinafterabbreviated as “PAI resins”) containing imide bonds and amide bonds intheir backbones.

[0197] In the present invention, a commercially-available PI resin orPAI resin can be conveniently used as the PI resin or PAI resin formolding the retainer. As commercially-available PI resins and PAIresins, the following resins can be listed including their chemicalstructural formulas.

[0198] (i) PI Resins:

[0199] (1) “P84-HT” (trade name; product of Lenzing AG, Austria;represented by the below-described chemical formula (1) in which Rrepresents an alkylene group);

[0200] (2) “TI-3000” (trade name; product of Toray Industries, Inc.;represented by the below-described chemical formula (2));

[0201] (3) “UIP-S” (trade name; product of Ube Industries, Ltd.;represented by the below-described chemical formula (3));

[0202] (4) “Vespel” (trade name; product of E.I. du Pont de Nemours &Co., Ltd.; represented by the below-described chemical formula (2));

[0203] (5) “Aurum” (trade name; product of Mitsui-Toatsu Chemicals Inc.;represented by the below-described chemical formula (4)); and

[0204] (6) Others, including “Meldin 8100” and “Meldin 900”, products ofFuron, U.S.A.

[0205] (ii) PAI Resins:

[0206] (1) “Torlon 4000 TF” (trade name; product of Amoco ChemicalCorp.; represented by the below-described chemical formula (5) in whichPh represents a phenylene group)

EXAMPLES

[0207] The present invention will hereinafter be described in furtherdetail by the following examples.

[0208] The lubricating oil according to the present invention, which iscomposed primarily of rice bran oil and is applicable to a rollingbearing of a dental air hand piece, was compared with various knownlubricating oils so as to substantiate the superiority of thelubricating oils of the present invention.

[0209] The rolling bearing of a dental air hand piece (see FIG. 1 andFIG. 2) furnished for use in the test of the example is a miniaturerolling bearing of the open type which is provided with a snap retainerand has the dimensions:

[0210] (i) Inner diameter of an outer ring: 6.350 mm

[0211] (ii) Inner diameter of an inner ring: 3.175 mm

[0212] (iii) Width: 2.380 mm

[0213] The above bearing unit was installed in a dental air turbine handpiece, and the test was conducted under the conditions of air supplypressure, 2.5 kgf/cm²; air supply rate, 26 l/min; and revolution speed,about 400,000 rpm.

[0214] Namely, using the dental air turbine hand piece (see FIG. 1 andFIG. 2), the lubricating oils according to the present inventionprimarily composed of rice bran oil and the various known lubricatingoils were tested under the above-described conditions. The test resultsare shown in Table 2.

[0215] In Table 2, the notes have the following meanings:

[0216] (1) <Heat Resistance Test (Autoclaving Resistance; Cycles)>

[0217] An autoclaving apparatus (“ALPHI”, trade name; manufactured by J.MORITA MFG. CORP.) was used. Autoclaving resistance is expressed interms of cycles until the rotation of the dental air turbine hand piecebecame unstable and the efficiency of rotation dropped to 10% (about40,000 rpm).

[0218] Conditions for the treatment in the autoclaving apparatus were:steam pressure, 2.4 kgf/cm²; temperature, 135° C.; time, 5 minutes.

[0219] (2) <Bearing Resistance (Continuous Operation; Hrs)>

[0220] Each lubricating oil was applied to a lubrication system of thedental air hand piece at the beginning, and a continuous operation wasthen performed at about 400,000 rpm without additional feeding of thelubricating oil. The bearing resistance is expressed in term of hoursuntil the rotation become unstable and the revolution speed dropped to10% (about 40,000 rpm).

[0221] The specific constructions of various retainers shown in Table 2are as follows:

[0222] (i) Non-Porous PI/PAI•R:

[0223] This means a non-porous retainer of the polyimide (PI) base orthe polyamideimide (PAI) base.

[0224] The non-porous PI•R was obtained by forming “Vespel SP-1” (tradename, product of E.I. du Pont de Nemours & Co., Ltd.) into aretainer-like shape.

[0225] On the other hand, the non-porous PAI•R was obtained by forming“Torlon 4203” (trade name, product of Teijin-Amoco Engineering Plastics,Ltd.) into a retainer-like shape.

[0226] (ii) Porous P•R:

[0227] This means a retainer of the phenol resin base.

[0228] Incidentally, the porous P•R was obtained by impregnatinginter-thread spaces of a woven fabric, which had been rolled pluraltimes into a pipe-like shape, with a phenol resin in a vacuum state,shaping the thus-impregnated fabric under heat and then machining theshaped fabric into a retainer-like shape.

[0229] (iii) Porous PI/PAI•R:

[0230] This means a porous retainer composed of a sintered body ofpolyimide (PI) base or polyamideimide (PAI) base powder.

[0231] The porous PI•R was obtained by compression-forming “UIP-S”(trade name, product of Ube Industries, Ltd.) under a forming pressureof 4000 kgf/cm², sintering the green compact at 400° C. in a nitrogengas atmosphere and then machining the sintered compact into aretainer-like shape (porosity: about 13% by volume).

[0232] The porous PAI-R was obtained by classifying and sifting “Torlon4000TF” (trade name, product of Amoco Chemical Corp., U.S.A.) into anaverage particle size of 20 μm, compression-forming the thus-shiftedpowder under a preforming pressure of 2,800 kgf/cm², sintering the greencompact at 300° C., and then machining the sintered compact into aretainer-like shape (porosity: about 14% by volume).

[0233] In Table 2,

[0234] (i) As the paraffin oil (liquid paraffin), a conventionalspray-type paraffin oil produced by an odontotherapy-related maker wasused.

[0235] (ii) As the fluorinated oil, “FOMBLIN” (trade name, product ofAusimont S.P.A., Italy) was used. TABLE 2 Evaluation of Characteristicsof Various Lubricating Oils Heat resistance (1) Bearing resistance (2)(autoclaving resistance, cycles) (continuous rotation, hrs) (i) (ii)(iii) (i) (ii) (iii) Bio- Environmental Non-porous Porous PorousNon-porous Porous Porous Lubricating Oil safery Conservation PI/PAI * RP * R PI/PAI * R PI/PAI * R P * R PI/PAI * R Vegetable Nondrying Ricebran oil A B 1300< 1300< 1500< 3.00 28.00 35.00 oil oil Refined oliveoil A B 1000< 1000< 1000< 1.50 25.00 29.75 Extra virgin oil A B 1000<1000< 1000< 2.00 23.50 25.50 Virgin olive oil A B 1000< 1000< 1000< 1.7516.50 18.25 Arachis oil A B 1000< 1000< 1000< 1.50 14.75 18.50Semidrying Rapeseed oil A B  300    300    300   0.25 2.25 4.50 oilMineral Paraffin oil C C 1000<  200   1000< 0.50 6.75 8.50 oil Parafin100 parts by weight + C C 1000<  150   1000< 1.00 4.75 17.00 acrylicresin 5 parts by weight Synthetic Fluorinated oil C C 1000< 1000< 1000<1.50 14.50 17.50 oil

[0236] As seen from Table 2, it is recognized that the lubricating oilaccording to the present invention which is composed primarily of ricebran oil and used in dental cutters of high-speed rotation (an airturbine hand piece) is excellent in the various characteristics ofbiosafety, environmental conservation, heat resistance (autoclavingresistance) and bearing durability compared to the conventionallubricating oils including nondrying vegetable oil such as olive oil,semidrying vegetable oil such as rapeseed oil, mineral oil and syntheticoil. Furthermore, the rice bran oil when applied in mist form has anexcellent characteristic to blood circulation in periodontal tissues andcleaning of cavity in tooth (or prevention of formation of sumea layer),and it is also excellent in economical efficiency compared to othernondrying vegetable oils.

[0237] Furthermore, the lubricating oil of the present invention isexcellent in economy because it costs lower than fluorinated oils.

[0238] Semidrying oils and drying oils, other than nondrying vegetableoils, and paraffin oil (liquid paraffin) show considerable color changeswhen tested or left over at 135° C. for 175 hours in air. In otherwords, these oils of different varieties are inferior in oxidationresistance as well. In the tests, nondrying oils such as rice bran oil,olive oil, arachis oil, etc. did not show color changes.

[0239] The lubricating oil according to the present invention which iscomposed primarily of rice bran oil and is applicable to a high-speeddental air turbine hand piece has a high biodegradation speed and, evenwhen evaluated based on the Interim Water Quality Standards forEffluents (an ordinance of the Office of the Prime Minister), is morepreferable than conventional mineral oils. Incidentally, according tothe Interim Water Quality Standards for Effluents, the permissible limitfor effluents is 30 mg/liter in the case of vegetable oils as opposed to5 mg/liter in the case of conventional mineral oils.

1. A lubricating oil for a rolling bearing used in a high-speed rotatingequipment, wherein said lubricating oil is comprised of a nondryingvegetable oil extracted from rice bran.
 2. The lubricating oil accordingto claim 1, wherein said nondrying vegetable oil extracted from ricebran comprises, as a component of glycerol ester thereof: 40-45 wt. % ofoleic acid (monovalent unsaturated fatty acid), 35-40 wt. % of linolicacid (divalent unsaturated fatty acid), 15-20 wt. % of palmitic acid(C₁₋₆ saturated fatty acid), and other element (balance (100 wt. %)). 3.The lubricating oil according to claim 2, wherein said nondryingvegetable oil extracted from rice bran comprises free fatty acid which,as a component other than glycerol ester of fatty acid, includes: lessthan 0.01 wt. % of free oleic acid, and less than 0.01 wt. % of freelinolic acid.
 4. The lubricating oil according to claim 2, wherein saidnondrying vegetable oil extracted from rice bran contains at least 500wt.ppm of α-tocophenol (vitamin E)
 5. The lubricating oil according toclaim 3, wherein said nondrying vegetable oil extracted from rice brancontains at least 500 wt.ppm of α-tocophenol (vitamin E)
 6. Alubricating oil for a rolling bearing used in a high-speed rotatingequipment, said lubricating oil comprising: 1) a nondrying vegetable oilextracted from rice bran, as a principal component; and 2) one type ofnondrying vegetable oil selected from the group consisting of olive oil,arachis oil and oleysol oil, as another component.
 7. The lubricatingoil according to any one of claims 1 to 6, wherein said nondryingvegetable oil contains oil-absorbing synthetic resin particles.
 8. Thelubricating oil according to claim 7, wherein said oil-absorbingsynthetic resin particles are made of crosslinked polymer obtained bypolymerizing a monomer mixture composed of: (A) 90 to 99.9 wt. % of amonomer composed of a monomer, which has a solubility parameter (spvalue) of at most 9, as a principal component, and containing onepolymerizable unsaturated group per molecule, and (B) 0.1 to 10 wt. % ofa crosslinking monomer having at least two polymerizable unsaturatedgroups per molecule.
 9. The lubricating oil according to claim 8,wherein said monomer (A) contains at least one C₃-C₃₀ aliphatichydrocarbon group and at least polymerizable unsaturated group selectedfrom the group consisting of residual groups of alkyl(meth)acrylates,alkylaryl(meth)acrylates, alkyl(meth)acrylamides,alkylaryl(meth)acrylamides, fatty acid vinyl esters, alkylstyrenes andα-olefins.
 10. The lubricating oil according to claim 7, wherein saidoil-absorbing synthetic resin particles are made of a diene-typecrosslinked polymer obtained by polymerization of a diene monomer.
 11. Alubricating oil for a rolling bearing used in a high-speed rotatingequipment, wherein said rolling bearing is provided with a rollingbearing element comprising an outer ring, an inner ring, rollingelements and a retainer to rotatably support a rotary shaft thereon towhich air turbine blades are fixed, and said lubricating oil used bybeing dropped to or impregnated in said retainer is comprised ofnondrying vegetable oil extracted from rice bran.
 12. The lubricatingoil according to claim 11, wherein said high-speed rotating equipment isa dental air turbine hand piece of a ball bearing type.
 13. Thelubricating oil according to claim 11 or 12, wherein said nondryingvegetable oil extracted from rice bran contains oil-absorbing syntheticresin particles.